[hbck2] Update hbck2 README to explain new "setRegionState" method.

HBase-22143 introduced new setRegionState method that allows for manually setting a region to an specific state, in meta. This was added a client side logic for updating region state, to workout case where region is left in OPENING state (unassigns/assigns will fail to progress here) on releases prior to 2.1.2 (when scheduleServerCrashProcedure was added to Master's HbckService). To reflect, this jira adds not only setRegionState description, but also additional explanations on the Overview section of README.md file.